Future wireless systems require a more effective utilization of the radio frequency spectrum in order to increase the data rate achievable within a given transmission bandwidth. This can be accomplished by employing multiple transmit and receive antennas combined with signal processing. A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station to improve the reliability of data communication over wireless media without compromising the effective data rate of the wireless systems. So called space-time block-codes (STBCs) are used to this end. Specifically, recent advances in wireless communications have demonstrated that, by jointly encoding symbols over time using multiple transmit antennas at a base station, one can obtain reliability (diversity) benefits as well as increases in the effective data rate from the base station to each cellular user. These multiplexing (throughput) gain and diversity benefits depend on the space-time coding techniques employed at the base station. The multiplexing gains and diversity benefits are also inherently dependent on the number of transmit and receive antennas in the system being deployed, in the sense that they are fundamentally limited by the multiplexing-diversity trade-offs curves that are dictated by the number of transmit and the number of receive antennas in the system.
A complimentary way of increasing the effectiveness/quality of transmission in the case of delivery of media, such as voice, audio, image and video, is to employ unequal error protection (UEP) methods, which are well-known in the art.
For high data rates and wideband transmission, the use of OFDM makes the equalizer unnecessary. With multilevel modems, coded modulation systems can easily be designed by use of an outer binary convolutional code and an interleaver in a so-called bit-interleaved coded modulation (BICM) system.
A large collection of STBCs have been proposed in recent years as a means of providing diversity and/or multiplexing benefits by exploiting multiple transmit antennas in the forward link of cellular systems. Given the presence of Nt transmit antennas, the typical objective is to design STBCs that provide order-“Nt” transmit-antenna diversity in the system. Typical STBC designs transmit an antenna-specific block of t samples per antenna for each block of k information symbols. Such STBC designs are described by a STBC matrix with t rows and n columns, whereby the (i, j)th entry denotes the sample transmitted by the antenna j at time i. Of interest is the symbol rate of the STBC scheme, R, which is equal to k/t (i.e., the ratio of k over t). Full rate STBCs are STBCs whose rate R equals 1 symbol per channel use. Another important attribute of a STBC is its decoding complexity. Although, the decoding complexity of the optimal decoder for arbitrary STBCs is exponential in the number k of jointly encoded symbols, there exist designs with much lower complexity. One such attractive class of designs, referred to as orthogonal space-time codes (OSTBCs), can provide full diversity while their optimal decoding decouples to (linear processing followed by) symbol-by-symbol decoding. Full rate OSTBCs exist only for a two transmit-antenna system. For three or more antennas the rate cannot exceed ¾ symbols/per channel use. This rate is achievable for Nt=3 and Nt=4 antennas. For more than four antennas the highest-rate OSTBCs are not known in general. In general, a rate equal to ½ symbols/channel use is always achievable, but, often, higher rates may also attainable for specific values of n.
A number of systems deployed for broadcasting common audio/video information from several base stations are exploiting coded OFDM transmission under the umbrella of the single frequency network concept. These systems employ a common coded OFDM-based transmission from each of the broadcasting base-stations. The OFDM based transmission allows asynchronous reception of the multitude of signals and provides increased coverage. However, as all base-stations transmit the same coded version of the information-bearing signal, single frequency network (SFN) systems do not provide in general full transmit base-station diversity with full coding gains (some form of this diversity is available in the form of multi-path diversity, although limited since it is not coordinated).
Another class of schemes are space-time bit-interleaved coded modulation (BICM) systems with OFDM and can provide spatial (transmit and receive antenna) diversity, frequency diversity and can cope with asynchronous transmission. Furthermore, by modifying the binary convolutional code to a rate compatible punctured convolutional code, a flexible UEP system can be achieved. For some systems, it is assumed that all transmit antennas are collocated at one and the same base station.
One drawback associated with the aforementioned BICM OFDM systems is that the near-optimum receiver can be quite complex (computation intensive). The necessary joint demapper unit (inner MAP decoder) grows in complexity exponentially with the product of the number of transmit antennas and the number of bits per modem constellation point. As an example with 16 QAM (4 bits/symbol) and transmit antennas, the complexity of the calculations in the inner decoder is proportional to 24×4=216 per block of 16 bits. There exist methods that can be used for reducing the decoder complexity without substantial loss in performance.
There exists a class of low complexity designs for narrowband transmission from multiple base stations to one or more receivers. These designs provide full transmit base-station diversity with very low decoding complexity even in the case of asynchronous reception. Although these designs can, in principle, also be employed for wideband transmission, and in fact some of these designs (the OFDM-type ones) still provide all the aforementioned benefits, they do not harvest any of the available frequency diversity available within the transmission bandwidth.